How to bake all sides of a model on a cuboid?
1: When this object is not shown closely in the game, then it is good enough to use a cubeoid. Performace issue might be only noticable when there are multiple disks on the scene, but this is not certain. Pushing a mesh with 50 polygons to the GPU istead 6 polygons will only cost minimal time, and rendering it out might be the same speed.

2: When the disk is shown more closely, the retopo might add realism to it. This looks like a mesh easy to retopo. You should whach some modeling tutorials. Starting from a cube, loop-cuting here and there, deforming the sucface, experiment with it.

3: More deeply below.

You can bake simply the way you baked the plane. In the examples,
the low-poly object is set to be displayed as wireframe. You set
up a UV mapping, texture to bake on, and then bake. :)




Note: You should use the shown unshaded material only in case, the entity and lighting will be static. In this case you could bake textures and shadows and save them as separate images and put them in the proper slot for the material, but Full Render might be also good in this case.

When the object is dinamic, or the lighting, then you should not use unshaded. Mix (multiply) two images, an abient occlusion, and the textures, and use that with a vertex-lit shader. This would be still pretty good performing. I also add the image with the normals to be complete, although you said that you do not need it.